Bleed-type thermostats regulate air pressure by bleeding control air to atmosphere at a varying rate in accordance with system demands. In variable speed fan drives, a fan motor having a variable sheave is moved with respect to the fan pulley which results in a different radius of contact between the belt and the sheave faces. As a result, fan output is related to fan motor position. The use of a bleed thermostat to supply control pressure for causing the movement of the fan motor, by itself, lacks a feedback to position the fan motor at an intermediate position in accordance with system demand.
In one type of pressure regulator a diaphragm divides the housing and is acted on by spring force and fluid pressure. Generally, a valve member is connected to the diaphragm and tends to be biased closed by fluid pressure acting on one side of the diaphragm. An opening bias is provided by a compression spring acting on the other side of the diaphragm with a force that is adjustable through a screw adjustment. Although such a pressure regulator is adjustable it must be manually adjusted. It is not automatically adjusted in response to system conditions.